In belt conveyor systems incorporating endless belts, tensioning devices of various kinds may be used to ensure that belt tension is always maintained within a target range along the belt path. In some cases, it may be desirable for the tensioning device (or “take-up device”) to continue maintaining belt tension within a target range, even after there has been a failure of the power supply to the conveyor drives and auxiliary equipment. In such cases, a tensioning or take-up device that includes some type of energy-storage means may satisfy the need.
Gravity-type take-up arrangements are the most common type of energy-storage system for conveyor take-ups. These types of energy storage systems often fall short of needs because they require significant space and are limited in their ability to modulate the tension response during transient tension waves that may arise after a sudden loss of power.
Some energy-storage systems are known, including those that use hydraulic accumulators to change the tension applied to a movable carriage (or a “take-up carriage”) after a power outage. For example, U.S. Pat. No. 4,007,826 describes a system where a conveyor take-up is tensioned for normal running by the pressure from a first (lower-pressure) hydraulic accumulator. During transient events that would otherwise result in localized excessive belt sag, the pressurization of the hydraulic take-up cylinder is switched to a second (higher-pressure) accumulator. The energy stored in the higher-pressure accumulator is then used to impose increased tension at the tail of the conveyor to avoid excessive sag arising during the rapid deceleration of the system. These types of systems are limited in that switching from the lower-pressure accumulator to the higher-pressure accumulator uses a powered switching circuit that requires power and/or power backup independent of the conveyor power system. The dual-pressure arrangement also adds complexity to the tensioning system.
Other tension management systems include “fully-active” or “semi-active” electric take-up winches that are increasingly used for high-power conveyors. These systems are limited in that “fully-active” or “semi-active” electric take-up winches cannot be called upon to actively change the tension applied to the take-up carriage during those dynamic events that follow a power outage, because power is also not available to drive the winch motor. Systems able to store sufficient electrical power to drive such winches for a short time immediately after a power outage would be costly and have not been available in the market.
There is thus a need for tensioning devices that will provide desirable behavior under both normal operating and “upset” conditions. In particular, during normal operation a tensioner should draw out the tensioning pulley with sufficient force and displacement such that when the conveyor belt is under full load, the tension provided by the tensioner remains at least in the target range. Likewise, when the conveyor belt is unloaded and contracted to a maximum degree, the tensioner should allow sufficient movement of the tensioning pulley so as to prevent excessive belt tensions. Further, during upset conditions (or “transient conditions”) following a loss of power, the tensioner should firstly provide for development of an increased level of tension that is nevertheless controlled within limits. Later during the transient conditions, the tensioner should be able to maintain a minimum level of tension by moving the tensioning pulley at sufficient speed and with enough force to absorb sag that might otherwise develop at some locations in the belt loop. Furthermore, the tensioning device should permit temporary application of higher static tensions to avoid slip when one of a conveyor's drive pulleys is out of order. The various examples of the present disclosure set forth below satisfy these needs.